Inductive power transfer

ABSTRACT

A detection method for use in a primary unit of an inductive power transfer system, the primary unit being operable to transmit power wirelessly by electromagnetic induction to at least one secondary unit of the system located in proximity to the primary unit and/or to a foreign object located in said proximity, the method comprising: driving the primary unit so that in a driven state the magnitude of an electrical drive signal supplied to one or more primary coils of the primary unit changes from a first value to a second value; assessing the effect of such driving on an electrical characteristic of the primary unit; and detecting in dependence upon the assessed effect the presence of a said secondary unit and/or a foreign object located in proximity to said primary unit.

The present invention relates to inductive power transfer methods,apparatuses and systems for use, for example, to power portableelectrical or electronic devices.

Inductive power transfer systems suitable for powering portable devicesmay consist of two parts:

-   -   A primary unit having at least one primary coil, through which        it drives an alternating current, creating a time-varying        magnetic flux.    -   A secondary unit, separable from the primary unit, having a        secondary coil.

When the secondary coil is placed in proximity to the time-varying fluxcreated by the primary coil, the varying flux induces an alternatingcurrent in the secondary coil, and thus power may be transferredinductively from the primary unit to the secondary unit.

Generally, the secondary unit supplies the transferred power to anexternal load, and the secondary unit may be carried in or by a hostobject (a secondary device) which includes the load. For example, thehost object may be a portable electrical or electronic device having arechargeable battery or cell. In this case, the load may be a batterycharger circuit for charging the battery or cell. Alternatively, thesecondary unit may be incorporated in such a rechargeable cell orbattery (secondary device), together with a suitable battery chargercircuit.

A class of such inductive power transfer systems is described inGB-A-2388716. A notable characteristic of this class of systems is thephysically “open” nature of the magnetic system of the primary unit; asignificant part of the magnetic path is through air. This permits theprimary unit to supply power to different shapes and sizes of secondaryunit, and to multiple secondary units simultaneously. Another example ofsuch an “open” system is described in GB-A-2389720. Although focus willnow be placed on such “open” and “multiple device” systems, this ismerely by way of example and it will be appreciated that the presentinvention may extend to all inductive systems, for example tosubstantially “closed” systems in which there is a near 1:1 relationshipbetween primary and secondary units with very little placement freedom.

Returning to “open” systems, such systems suffer from a number ofproblems. A first problem is that the primary unit cannot be 100%efficient. For example, switching losses in the electronics and I²Rlosses in the primary coil dissipate power even when there is nosecondary unit present, or when no secondary units that are presentrequire charge. This may waste energy so it may be desirable for theprimary unit to enter a low-power “standby mode” in this situation.

A second problem in such systems is that it is generally not possible tomechanically prevent foreign objects (made of metal) from being placedinto proximity with the primary coil and coupling to the coil. Foreignobjects made of metal may have eddy-currents induced therein. Such eddycurrents tend to act to exclude the flux, but because the material hasresistance, the flowing eddy currents may cause I²R losses that maycause heating of the object.

There are, perhaps among others, two particular cases where this heatingmay be significant:

-   -   If the resistance of any metal is high, for example if it is        impure or thin.    -   If the material is ferromagnetic, for example including steel.        Such materials have high permeability, encouraging a high flux        density within the material, and causing large eddy currents and        therefore large I²R losses.

In the present application, such foreign objects that cause power drainare termed “parasitic loads”. Optionally, the primary unit could enter a“shutdown mode” when parasitic loads are present, to avoid heating them.

Various approaches to solving these two problems have been previouslyconsidered.

Previously-considered approaches to solving the first problem, of notwasting power when no secondary unit requires charge, include thefollowing:

-   -   In a first approach, the secondary unit modulates its inductive        load during charging, causing a corresponding variation in the        power taken from the primary unit so as to transmit information        back to the primary unit. This indicates to the primary unit        that it should stay out of the standby state.    -   In a second approach, the primary unit determines whether a        secondary unit or foreign object is present based upon changes        in the current flowing in and/or the voltage appearing across        the primary coil. If a secondary unit requiring charge is        detected, the primary unit may stay out of the standby state.    -   In a third approach, the primary unit varies the frequency of        its drive, and thus the coupling factor to a tuned secondary        unit (i.e. due to resonance). If the secondary unit is not        taking power, there is no substantial difference in the power        taken as the frequency is swept and thus the primary unit goes        into a standby state.    -   In a fourth approach, the primary unit measures the power        flowing in the primary coil, and enters a pulsing standby state        if this is below a threshold.    -   In a fifth approach, the primary unit contains detecting coils        which have power coupled back into them according to the        position of the secondary unit. If the device is not present,        the primary unit enters a standby mode.    -   In a sixth approach, the secondary unit has a mechanical        protrusion that fits a slot in the primary unit, activating it.    -   In a seventh approach, the primary unit drives two coils, and        there are a corresponding two power receiving secondary coils in        the secondary unit. The primary unit measures the power        delivered from each primary coil and enters standby mode if it        is below a threshold.    -   In an eighth approach, the primary unit includes a resonant tank        and control circuitry which serves to maintain slightly more        energy in the resonant tank than is needed to supply power over        the inductive link. If demand for power drops off, the control        circuitry serves to shut down further build up of energy in the        resonant tank.    -   In a ninth approach, the primary unit includes a comparator that        detects the average current of the primary coils and compares        that to a reference. If the average current is below the        reference level, it is assumed that the primary unit is in the        no-load state.    -   In a tenth approach, the primary unit places the primary coil        into an undriven resonating condition during a measurement phase        such that it acts as a resonant tank, and measures the decay of        energy in the resonant tank to determine how much energy is        being removed from it. The secondary units are caused to place a        different load on the primary coil for different such        measurement phases, and thus differences in power drawn from the        primary unit will be detected if a secondary unit is present. In        the absence of a secondary unit, the primary unit may enter a        standby mode.    -   In an eleventh approach, the secondary unit(s) are set into a        no-load state during a measurement phase, during which time the        primary unit measures power drawn from its primary coil. If the        power drawn from the primary coil does not substantially change        as the measurement phase is entered, it is assumed that no        secondary unit requiring power is present and the primary unit        may enter a standby mode.    -   In a twelfth approach, the secondary unit(s) communicate        information relating to their power requirement to the primary        unit. The primary unit then measures power drawn from its        primary coil and compares this to the power requirement. If no        such information is received, the primary unit may determine        that no secondary unit is present and may enter a standby mode.

Previously-considered solutions to the second problem, of parasiticloads, include:

-   -   In a thirteenth approach, the primary unit varies the frequency        of its drive. In this system, the secondary unit is tuned, so        this frequency variation will result in a variation of the power        taken from the primary unit. If the load is instead a piece of        metal, then varying the frequency will not have a substantial        effect and the primary unit will enter a shutdown state.    -   In a fourteenth approach, a key in the secondary unit activates        the primary unit. The assumption is that if a secondary unit is        present then this will physically exclude any foreign objects.    -   In a fifteenth approach, the primary unit supplies power to the        secondary unit by driving two primary coils. If the amount of        power supplied by the two coils is different, the primary unit        assumes that the load is not a valid secondary unit and enters        shutdown mode.    -   In a sixteenth approach, the primary unit includes comparators        which are used to detect imbalances between the voltages and        currents in the coils of the primary unit. Such imbalances are        considered to indicate detection of a foreign body.    -   In a seventeenth approach, the secondary unit modulates its        inductive load during charging, causing a corresponding        variation in the power taken from the primary unit so as to        transmit information back to the primary unit. The assumption is        that if such information is not received in the primary unit,        either no secondary unit is present or a foreign object is        present.    -   In an eighteenth approach, the primary unit determines whether a        secondary unit or foreign object is present based upon changes        in the current flowing in and/or the voltage appearing across        the primary coil. If a foreign object is detected, the primary        unit may enter shutdown mode.    -   In a nineteenth approach, the primary unit places the primary        coil into an undriven resonating condition during a measurement        phase. A series of measurement phases are carried out during        which the secondary unit(s) place different loads on the primary        coil. The measurement phases are configured such that the        primary unit can determine whether or not an unexpected        parasitic load (i.e. a foreign object) is present. If it is        determined that a foreign object is present, the primary unit        may enter shutdown mode.    -   In a twentieth approach, the secondary unit(s) are set into a        no-load state during a measurement phase, during which time the        primary unit measures power drawn from its primary coil. If the        power drawn from the primary coil during the measurement phase        is, for example, above a threshold, it is determined that a        foreign object is present and the primary unit may enter a        shutdown mode.    -   In a twentyfirst approach, the secondary unit(s) communicate        information relating to their power requirement to the primary        unit. The primary unit then measures power drawn from its        primary coil and compares this to the power requirement. If the        power drawn exceeds the power required by more than a threshold        value, it is determined that a foreign object is present and the        primary unit may enter a shutdown mode.

A number of these approaches assume a 1:1 relationship between theprimary unit and the secondary unit. Therefore, they are not sufficientfor systems such as those described in GB-A-2388716 where more than onesecondary unit (or secondary device) at a time may be present. Forexample, they may not work when there are two secondary units present,one requiring charge and the other not.

Some of these approaches are not able to cope with a foreign object(e.g. a piece of metal) in the presence of valid secondary units. Anumber of those approaches assume that the physical or electricalpresence of a valid secondary unit implies that all foreign objects arephysically excluded by the secondary unit. This is not necessarily thecase, particularly when the secondary units may be positionedarbitrarily in respect of the primary unit, as in those described inGB-A-2388716.

Some of these approaches are undesirable in terms of EMC(Electromagnetic Compatibility) performance. For example, the third andtenth approaches above involve frequency variations, and such variationscan cause interference with other systems. Typically, inductive devicesare designed to operate within an assigned frequency “chimney” or“window”, i.e. within a certain frequency range separate from frequencyranges used by other systems. By fluctuating or varying frequencies ofoperation, inductive systems can have increased risk of falling foul ofEMC requirements. These approaches will therefore be considered in moredetail to highlight their possible disadvantages.

In the third approach, the system is resonant on the secondary side, butnot resonant on the primary side. Thus, when a valid secondary device isplaced in proximity to the primary unit, the overall circuit will have aresonant frequency. Consequently, changing the driving frequency willchange the power delivered to the secondary side, and also, therefore,the current in the primary sense resistor. When there is no validsecondary device present, the system is not resonant, and therefore achange in driving frequency may, to a first order, not have asignificant effect.

Not having a resonant primary side may be disadvantageous. Without thecapacitance to oppose the impedance of the inductor, there is a highimpedance load which is difficult to drive. The system may thus beinefficient. In a resonant system, energy is cycled between the inductorand the capacitor as the instantaneous voltage changes. Without thecapacitance, energy that flows out of the inductor will simply bedissipated in the driver electronics.

If a resonant capacitor was added to the primary coil, then the systemmay not function properly, as follows. If a non-resonant foreign bodywas in proximity to the primary side, there would still be a change inpower delivered and also sense current with frequency, because of theresonant primary side. The foreign body may change the inductance andresonant frequency in a non-predictable way. The primary circuit couldbe made resonant without a capacitor on the primary coil if there was avery strong coupling between the primary and secondary coils, by virtueof a capacitor on the secondary. However, this is not practical forcontactless systems in which there is air in at least part of themagnetic circuit.

The previously-considered third approach uses a 10% frequency modulationto get sufficient signal to noise ratio. 10% is actually a very largechange in frequency in spectral terms. Typically, there are certain“chimneys” where radiation is allowed and they may not be wide enough toaccommodate this frequency variation. Another consideration is that themodulation itself may generate multiple harmonics which may extend along way out in frequency, either side of the fundamental. The change infrequency also results in lower power delivered during some timeintervals; the load may have to be able to cope with this periodicreduction in available power.

Turning to the previously-considered tenth approach, the systemfunctions by allowing the primary unit to momentarily stop deliveringpower to the primary coil. This allows the energy in the system to decayto zero, and by measuring the rate of decay it is possible to measurethe losses in the system. Three measurements are made to isolateparasitic losses from load losses and ‘friendly parasitic losses’ (e.g.metal components included in a valid secondary unit). A key disadvantageof this particular approach is that power is suspended, i.e.measurements are taken in the undriven state when power is not activelybeing transferred. It is therefore desirable to have a large capacitorin the secondary device. Such a capacitor may be physically large, andtherefore undesirable for integration into modern secondary devices suchas mobile telephones. However, one disadvantage is that switching thepower off suddenly causes a transient, resulting in a broad spectrum offrequencies (cf. Fourier transform of a step response). As the energydecays, cycling between the inductor and capacitor, this energy isradiated from the primary coil, which may be exposed in an open system.This broad spectrum of radiated power may be problematic for EMC.

The previously-considered twentieth approach in which the secondaryunit(s) are set into a no-load state during a measurement phase, canrequire the use of a large hold-up capacitor in each secondary unit tomaintain power transfer during the measurement phases. This can bedisadvantageous in terms of cost and size.

The previously-considered twentyfirst approach in which the secondaryunit(s) communicate information relating to their power requirement tothe primary unit requires a communication link to be implemented. Thiscan be complex technically and, for example, may require a degree ofcollision-detection if multiple secondary units are present.

It is desirable to solve some or all of the above-mentioned problems.

According to an embodiment of a first aspect of the present invention,there is provided a detection method for use in a primary unit of aninductive power transfer system, the primary unit being operable totransmit power wirelessly by electromagnetic induction to at least onesecondary unit of the system located in proximity to the primary unitand/or to a foreign object located in said proximity, the methodincluding: driving the primary unit so that in a driven state themagnitude of an electrical drive signal supplied to one or more primarycoils of the primary unit changes from a first value to a second value;assessing the effect of such driving on an electrical characteristic ofthe primary unit; and detecting in dependence upon the assessed effectthe presence of a said secondary unit and/or a foreign object located inproximity to said primary unit.

The electrical characteristic of the primary unit may be a level ofpower being drawn from the primary unit, or for example a characteristicthat varies as a function of the level of power being drawn.

In embodiments of the present invention, assessments (e.g. measurements)are made in the driven state, as opposed to in the undriven state. Thatis, it may not be necessary to suspend power transfer, and sizeablestorage capacitors in the secondary unit may not be required.Furthermore, embodiments of the present invention are not dependent onfrequency-variation techniques, which is advantageous both in terms ofsystem capacity, and in terms of EMC performance. A “driven state” maybe interpreted to mean when drive signals are actively supplied, ratherthan passively supplied. A primary unit may enter such a driven stateeven if it is in a “standby” or “shutdown” mode, for example bytemporarily supplying drive signals actively.

Advantageously, by driving the primary unit in this way, it is possibleto detect one or more secondary units and/or foreign objects inproximity to the primary unit. That is, the present method enables a“1:many” relationship between the primary unit and the secondary unitsand/or foreign objects to be handled, as well as a 1:1 relationship.

Such a method involves driving the primary unit so that a signalmagnitude substantially changes. Such a change is desirably substantial(i.e. of substance) so that noise or other minor variations in powerdrawn are compensated for. One possible way of driving the primary unitin this way is to change a voltage level across one or more primarycoils of the primary unit, as will become apparent below.

Such a method may be adapted to detect the presence of, and optionallydifferentiate between, secondary units requiring and not requiringpower. Such a method may be suitable for controlling inductive powertransfer in such an inductive power transfer system, and may includerestricting or stopping the inductive power supply from the primary unitif a foreign object is detected, and/or if no secondary unit requiringpower is detected.

Such driving includes controlling the primary unit so as to change themagnitude of an electrical drive signal supplied to one or more primarycoils of the primary unit from a first value to a second value. Changingthe magnitude of drive signals is a relatively straightforward drivingmethod, which is therefore advantageous in terms of cost and complexity.In contrast, changing other parameters in respect of the drive signalscan be complex and have undesirable side effects. For example, changingthe frequency of the drive signals can result in poor EMC(Electromagnetic Compatibility) performance. In one embodiment, it ispossible to slowly ramp up or down the magnitude of the drive signals,rather than change the magnitude in a stepwise fashion, so as to avertproblems with transients.

The first and second values may characterise the electrical drivesignal. For example, the drive signal may be a fluctuating oralternating signal, whose magnitude naturally changes over time, andsuch values may characterise the fluctuating or alternative signal (forexample in terms of its peak value or RMS value).

Such values may be peak values or root-mean-square values of analternating potential difference supplied across one or more primarycoils of the primary unit. Similarly, such values may be peak values orroot-mean-square values of an alternating current passing through one ormore primary coils of the primary unit. These types of values can berelatively easy to control and maintain.

Signals in a primary coil of such a primary unit can take time to settleif their magnitudes are changed. Optionally, therefore, the methodincludes maintaining the first and second values steady for long enoughfor operation of the primary unit to stabilise.

The second value may be larger than the first value, or, conversely, thesecond value may be smaller than the first value. The second value maybe between 5 and 50% (for example, 10%) larger or smaller than the firstvalue. In one embodiment, it may be desirable for the second value to belarger than the first value, to aim to cause an increase in the amountof power drawn, for example in the case that a foreign object ispresent. Power regulation in the secondary unit may be by means of aBuck regulator, whose operation is generally more efficient when itsinput voltage is closer to its output voltage. A Buck regulator cangenerally only down-convert the voltage. Boost converters, which canupconvert, are generally less efficient than Buck converters. By aimingto increase the amount of power drawn, i.e. by boosting the voltageduring the measurement phase, the voltage will be at the lower voltagefor most of the time, and the efficiency will be better. In oneembodiment, it may be desirable to use a Boost converter rather than aBuck converter/regulator.

The primary unit may include DC-AC conversion means (for example aninverter or other DC-to-AC converter) for converting DC electrical drivesignals into time-varying electrical drive signals for supply to the oneor more primary coils concerned. In that case, such driving may includecontrolling operation of the conversion means.

Operation of the conversion means may be governed by a duty cycle, inwhich case the driving may include controlling the duty cycle of theDC-AC conversion means.

The primary unit may also include DC-DC conversion means for outputtingthe DC electrical drive signals in dependence upon DC input signals. Inthat case, such driving may include controlling the operation of theDC-DC conversion means. Operation of the DC-DC conversion means may begoverned by a duty cycle, in which case the driving may includecontrolling the duty cycle of the DC-DC conversion means.

It may be more preferable to control the DC-DC conversion means than theDC-AC conversions means. A non-50% duty cycle in the DC-AC conversionmeans may result in even and odd harmonics. An even (e.g. 2^(nd))harmonic may be filtered less well by a resonant circuit than an oddharmonic because it is closer-in in frequency range.

Such driving may be considered to include reconfiguring operation of theprimary unit from an existing state preceding the change to a changedstate succeeding the change, both states being driven states of theprimary unit. Such assessment may therefore include obtaining ameasurement of the level of an electrical characteristic of the primaryunit in the existing state and in the changed state, such as the levelof power being drawn or a characteristic that varies as a function ofthe level of power being drawn. The method may include maintaining thefirst value during the existing state and maintaining the second valueduring the changed state. This may advantageously involve ensuring thatthe first and second values are maintained for long enough to obtainvalid measurements.

Such assessment may include taking voltage and current measurements inrespect of primary-coil signals. Such measurements need not be directlytaken at the primary coil, and may for example be voltage and/or currentmeasurements in respect of the DC electrical drive signals mentionedabove. The measurements may be directly taken at the primary coil, or atleast on the AC side of such conversion means, in which case they may bevoltage and current measurements in respect of the time-varyingelectrical drive signals mentioned above.

The method may involve taking a series of samples of said voltages andcurrents, and basing such assessment on the series of samples. Suchsamples may be averaged or combined in some other way to improve thereliability of such assessment. For further such improvement, thedriving and assessing may be considered to form a set of method steps,and the method may include carrying out a plurality of such sets andbasing such detection on two or more of such sets. In this way, furtheraveraging may be employed.

It may be determined that a said foreign object is present in proximityto said primary unit if it is determined that the electricalcharacteristic of the primary unit has substantially changed as a resultof such driving. A foreign object (e.g. a set of keys or a lump ofmetal) may draw substantially more power if (for example) the voltageover the primary coil(s) is substantially increased, or drawsubstantially less power if that voltage is substantially decreased.

The or each said secondary unit of the system is optionally configuredsuch that, when in proximity to the primary unit and receiving powerinductively therefrom, an electrical characteristic of that secondaryunit responds to such driving in an expected manner (e.g. it is aregulated secondary device), the method further including determiningwhether a said secondary unit and/or a foreign object is present inproximity to said primary unit in dependence upon results of suchassessment and the or each such expected response. For example, theresults may be compared with the or each expected response.

For the or each secondary unit the electrical characteristic of thatsecondary unit may be its power drawn from the primary unit, or forexample a characteristic that varies as a function of its power drawnfrom the primary unit.

It may be determined that a foreign object is present if the results ofthe assessment at least partly do not correlate with (or correspond to,or map to, or bear the signature of) the or any said expected response.It may be determined that a secondary unit is present if the results ofthe assessment at least partly do correlate with the or one saidexpected response. It may be that, for the or at least one saidsecondary unit, the expected response is that its electricalcharacteristic does not substantially change in response to suchdriving. For example, the secondary units may be regulated, such thatthey draw the same amount of power from the primary unit as long as thatamount of power is available.

The expected response for one said secondary unit may be different fromthe expected response for another such secondary unit. Such expectedresponses may be different because the secondary units concerned are ofdifferent types, and/or because the secondary units are incorporated insecondary devices of different types.

With this in mind, the method may further include, in the primary unit,receiving from the or each secondary unit that is in a power requiringstate, information relating to the expected response for the secondaryunit concerned. The information need not directly detail the relevantexpected response. For example, the information may merely identify thetype of secondary unit concerned, and the method may further includedetermining the expected response based upon the identified type ofsecondary unit concerned. For example, the primary unit may store (orhave access to) information detailing expected responses for differenttypes of secondary unit.

The method optionally includes employing, when carrying out thedetection, secondary-unit compensation information relating to aparasitic load imposed on the primary unit by the or each secondary unitso as to compensate for said parasitic load of the or each secondaryunit. For example, in this way it may be possible to compensate formetal (or some other parasitic load) present in the secondary unit(s)that is expected to be present and is thus unavoidable. Without thiscompensation, it is possible that substantial parasitic load in thesecondary unit(s) may be detected as constituting a foreign body. Suchsecondary-unit compensation information may be received by the primaryunit from the or each secondary unit.

Part or all of such information received in the primary unit from one ormore secondary unit may be received via a communication link separatefrom a link constituted by the transfer of inductive power, for exampleover an RFID link or some other separate communications link, radio orotherwise. For example, infrared or ultrasound communication may beused. Part or all of such information may be received via an inductivecommunication link constituted by the transfer of inductive power. Anycombination of communication links may be employed.

The method may further include employing, when carrying out suchdetection, primary-unit compensation information relating to losses inthe primary unit itself so as to compensate for said losses. Forexample, in this way it may be possible to compensate for metal (or someother parasitic load) present in the primary unit that is expected to bepresent and is thus unavoidable. Without this compensation, it ispossible that substantial parasitic load in the primary unit itself maybe detected as constituting a foreign body. Part or all of suchprimary-unit compensation information may be derived from measurementstaken by the primary unit when it is effectively in electromagneticisolation.

Following detection of a foreign object in proximity to the primaryunit, the method may include restricting or stopping inductive powersupply from the primary unit. This may be referred to as entering a“shutdown” mode of operation. Following detection of one or moresecondary units requiring power, the method may include maintaining oradjusting inductive power supply from the primary unit to meet suchrequirement. This may be referred to as entering an “operating” or“normal” mode of operation. Following detection of one or more secondaryunits not requiring power in the absence of one or more secondary unitsrequiring power, the method may include restricting or stoppinginductive power supply from the primary unit. This may be referred to asentering a “standby” mode of operation.

It may be desirable to detect a condition for entering a standby mode inaddition to detecting when to enter the shutdown mode. For example, inthe method of the first aspect it is possible to restrict or stop theinductive power supply in the event that the expected behaviour of asecondary unit requiring power is not detected, which may be the resultof all present secondary units not requiring power (rather than theresult of a foreign object being present). The primary unit may beconfigured to require user input to exit shutdown mode, but not requireuser input to exit standby mode.

The present invention may lend itself in some embodiments to driving asingle primary coil, and in other embodiments to driving a plurality ofdifferent primary coils. For example, one embodiment may includeswitching between one primary coil driven with a signal of a firstmagnitude to a second primary coil driven with a signal of a secondmagnitude, or for example switching between driving a first number ofprimary coils (e.g. one) and driving a second number of primary coils(e.g. two or more) simultaneously, the second number being differentfrom the first number.

According to an embodiment of a second aspect of the present invention,there is provided a primary unit for use in an inductive power transfersystem, the primary unit being operable to transmit power wirelessly byelectromagnetic induction to at least one secondary unit of the systemlocated in proximity to the primary unit and/or to a foreign objectlocated in said proximity, the primary unit including: driving means(e.g. driving circuitry) operable to drive the primary unit so that anamount of power drawn from the primary unit by a purely-resistive loadin proximity thereto would substantially change; means (e.g. circuitry)for assessing the effect of such driving on an electrical characteristicof the primary unit; and means (e.g. circuitry) for detecting independence upon the assessed effect the presence of a said secondaryunit and/or a foreign object located in proximity to said primary unit.

According to an embodiment of a third aspect of the present invention,there is provided an inductive power transfer system, including aprimary unit and at least one secondary unit, the primary unit beingoperable to transmit power wirelessly by electromagnetic induction to atleast one said secondary unit located in proximity to the primary unitand/or to a foreign object located in said proximity, the systemincluding: driving means (e.g. driving circuitry) operable to drive theprimary unit so that an amount of power drawn from the primary unit by apurely-resistive load in proximity thereto would substantially change;means (e.g. circuitry) for assessing the effect of such driving on anelectrical characteristic of the primary unit; and means (e.g.circuitry) for detecting in dependence upon the assessed effect thepresence of a said secondary unit and/or a foreign object located inproximity to said primary unit.

According to an embodiment of a fourth aspect of the present invention,there is provided a computer program which, when executed on a computingdevice of a primary unit, causes the primary unit to carry out adetection method according to the aforementioned first aspect of thepresent invention. Such a computer program may be stored on any suitablecarrier medium, and may be transmitted as a carrier signal over acommunication link, such link for example being part of the Internet.

According to an embodiment of a fifth aspect of the present invention,there is provided a system for transferring power from a primary unit toa secondary unit separable from the primary unit by electromagneticinduction; the primary unit including: a primary coil; an alternatingvoltage or current source coupled to the primary coil; means (e.g.circuitry) for adjusting the voltage or current of the primary coilbetween at least two levels; means (e.g. circuitry) for determining thepower drawn by the primary coil; the secondary unit including: asecondary coil; a voltage or current converter; wherein said voltage orcurrent regulator acts such that the power drawn from the secondary coilis a known function of voltage or current input level; wherein theprimary unit measures the power drawn by the primary coil in at leasttwo primary coil voltage or current levels and in dependence stops orrestricts power to the primary coil.

According to an embodiment of a sixth aspect of the present invention,there is provided a primary unit for transferring power to a secondaryunit separable from the primary unit by electromagnetic induction; theprimary unit including: a primary coil; an alternating voltage orcurrent source coupled to the primary coil; means (e.g. circuitry) foradjusting the voltage or current of the primary coil between at leasttwo levels; means (e.g. circuitry) for determining the power drawn bythe primary coil; wherein the primary unit measures the power drawn bythe primary coil in at least two primary coil voltage or current levelsand in dependence stops or restricts power to the primary coil.

According to an embodiment of a seventh aspect of the present invention,there is provided a method for transferring power from a primary unit toa secondary unit separable from the primary unit by electromagneticinduction; the method including the steps of: supplying alternatingcurrent or voltage to a primary coil in the primary unit; taking a firstmeasurement of the power drawn in the primary unit; changing themagnitude of the current or voltage supplied to the primary unit; takinga second measurement of the power drawn in the primary unit; independence of the results of the first and second measurement stoppingor restricting the magnitude of the alternating current or voltagesupplied to the primary coil in the primary unit.

According to an embodiment of an eighth aspect of the present invention,there is provided a system for transferring power from a primary unit toa secondary unit separable from the primary unit by electromagneticinduction; the primary unit including: a primary coil; an alternatingvoltage or current source coupled to the primary coil; means (e.g.circuitry) for adjusting the voltage or current of the primary coilbetween at least two levels; means (e.g. circuitry) for determining thepower drawn by the primary coil; the secondary unit including: asecondary coil; a voltage or current converter; wherein said voltage orcurrent converter acts such that the power drawn from the secondary coilis substantially independent of the voltage or current at the secondarycoil; wherein the primary unit measures the power drawn by the primarycoil in at least two primary coil voltage or current levels and stops orrestricts power if there is a substantial difference.

According to an embodiment of a ninth aspect of the present invention,there is provided a primary unit for transferring power to a secondaryunit separable from the primary unit by electromagnetic induction; theprimary unit including: a primary coil; an alternating voltage orcurrent source coupled to the primary coil; means (e.g. circuitry) foradjusting the voltage or current of the primary coil between at leasttwo levels; means (e.g. circuitry) for determining the power drawn bythe primary coil; wherein the primary unit measures the power drawn bythe primary coil in at least two primary coil voltage or current levelsand stops or restricts power if there is a substantial difference.

According to an embodiment of a tenth aspect of the present invention,there is provided a method for transferring power from a primary unit toa secondary unit separable from the primary unit by electromagneticinduction; the method including the steps of: supplying alternatingcurrent or voltage to a primary coil in the primary unit; taking a firstmeasurement of the power drawn in the primary unit; changing themagnitude of the current or voltage supplied to the primary unit; takinga second measurement of the power drawn in the primary unit; stopping orrestricting supply of current or voltage to the primary coil in theprimary unit if there is a substantial difference between the first andsecond measurement.

According to an embodiment of an eleventh aspect of the presentinvention, there is provided a detection method for use in a primaryunit of an inductive power transfer system, the primary unit beingoperable to transmit power wirelessly by electromagnetic induction to atleast one secondary unit of the system located in proximity to theprimary unit and/or to a foreign object located in said proximity, themethod including: driving the primary unit so that an amount of powerdrawn from the primary unit by a purely-resistive load (or anunregulated load, or a test unit including substantially only apurely-resistive load, or a load that is non-resonant at frequencies inthe vicinity of a frequency of operation of the primary unit) inproximity thereto would substantially change; assessing the effect ofsuch driving on an electrical characteristic of (e.g. a level of powerbeing drawn from) the primary unit; and detecting in dependence upon theassessed effect the presence of a said secondary unit and/or a foreignobject located in proximity to said primary unit.

According to an embodiment of a twelfth aspect of the present invention,there is provided a voltage and/or current-mode detection method for usein a primary unit of an inductive power transfer system, the primaryunit being operable to transmit power wirelessly by electromagneticinduction to at least one secondary unit of the system located inproximity to the primary unit and/or to a foreign object located in saidproximity, the method including: driving the primary unit so that anamount of power drawn from the primary unit by a purely-resistive load(or an unregulated load, or a test unit including substantially only apurely-resistive load, or a load that is non-resonant at frequencies inthe vicinity of a frequency of operation of the primary unit) inproximity thereto would substantially change; assessing the effect ofsuch driving on an electrical characteristic of (e.g. a level of powerbeing drawn from) the primary unit; and detecting in dependence upon theassessed effect the presence of a said secondary unit and/or a foreignobject located in proximity to said primary unit.

According to an embodiment of a thirteenth aspect of the presentinvention, there is provided a detection method for use in a primaryunit of an inductive power transfer system, the primary unit beingoperable to transmit power wirelessly by electromagnetic induction to atleast one secondary unit of the system located in proximity to theprimary unit and/or to a foreign object located in said proximity, themethod including: driving the primary unit so that in a driven state anamount of power drawn from the primary unit by a purely-resistive load(or an unregulated load, or a test unit including substantially only apurely-resistive load, or a load that is non-resonant at frequencies inthe vicinity of a frequency of operation of the primary unit) inproximity thereto would substantially change; assessing the effect ofsuch driving on an electrical characteristic of (e.g. a level of powerbeing drawn from) the primary unit; and detecting in dependence upon theassessed effect the presence of a said secondary unit and/or a foreignobject located in proximity to said primary unit.

Further aspects (and thus embodiments) of the present invention areenvisaged, for example being one of the aforementioned aspects in whichthe secondary unit has a voltage regulator, a current regulator, acombination of voltage and current regulation, or a power regulator.

Method aspects may apply by analogy to primary-unit aspects, systemaspects and computer-program aspects, and vice versa.

Reference will now be made, by way of example, to the accompanyingdrawings in which:

FIG. 1 is a schematic diagram of elements of an inductive power transfersystem according to one embodiment of the present invention;

FIG. 2 is a flow chart representing a method according to one embodimentof the present invention;

FIG. 3 is a another schematic diagram of the FIG. 1 system 1;

FIG. 4 is a schematic diagram illustrating the different modes ofoperation in the FIG. 1 system and the conditions for switching betweenthese different modes;

FIGS. 5(A) to 5(E) illustrate the conditions under which the modes ofFIG. 4 are selected;

FIG. 6 is a schematic diagram of another system according to oneembodiment of the present invention;

FIG. 7 shows equivalent circuits of a primary coil and resonantcapacitor to show an effect that a foreign body may have on anequivalent circuit seen by a primary coil;

FIG. 8 shows a set of three graphs respectively showing waveforms forthe voltage V_(d), the current I_(d), and the power drawn P;

FIGS. 9 to 11 show graphs similar to those shown in FIG. 8;

FIG. 12 is an enlarged version of FIG. 8( a), intended to provide anexample as to when measurements could be taken;

FIGS. 13( a) to 13(c) show timing diagrams for the FIG. 6 system underdifferent conditions;

FIG. 14 is a flow diagram of another method according to one embodimentof the present invention;

FIG. 15 is a schematic diagram of another system according to oneembodiment of the present invention;

FIG. 16 shows a typical current and voltage profile for charging aLithium Ion battery;

FIG. 17 is a schematic diagram of a secondary unit which may besubstituted for the secondary unit in the systems;

FIGS. 18 and 19 are schematic diagrams of further secondary units;

FIG. 20 is a schematic diagram of another primary unit according to oneembodiment of the present invention;

FIGS. 21 to 23 are schematic diagrams of primary units 810, 910 and1010, respectively;

FIGS. 24 and 25 are schematic diagrams of possible coil layouts on thecharging surfaces of primary units according to some embodiments of thepresent invention; and

FIG. 26 is a schematic diagram of a primary unit according to oneembodiment of the present invention.

FIG. 1 is a schematic diagram of elements of an inductive power transfersystem 1 according to one embodiment of the present invention. Thesystem 1 includes a primary unit 10 and at least one secondary unit 20.The primary unit 10 itself also embodies the present invention.

The primary unit 10 includes a primary coil 12 and an electrical driveunit 14 connected to the primary coil 12 for applying electrical drivesignals thereto, so as to generate an electromagnetic field. A controlunit 15 is connected to the electrical drive unit 14, and includes anadjustment unit 16, an assessment unit 17 and a detection unit 18.

The adjustment unit 16 is connected to the electrical drive unit 14 toadjust or control the electrical drive signals, or at least oneelectrical drive signal, applied to the primary coil 12. The assessmentunit 17 is connected to the electrical drive unit 14 to assess theamount of power drawn from the primary coil via the generatedelectromagnetic field. The detection unit 18 is connected to theassessment unit 17, to detect, in dependence upon the assessment made bythe assessment unit 17, the presence of entities in proximity to theprimary unit, as discussed further below.

The detection unit 18 is optionally connected to the adjustment unit 16to enable the electrical drive signals applied to the primary coil 12 tobe controlled in dependence upon the detection. For example, the mode ofoperation of the primary unit 10 could be controlled in dependence uponthe detection, for example to place the primary unit 10 into one of“charging”, “standby” and “shutdown” modes of operation.

As mentioned above, the primary unit 10 is configured to generate anelectromagnetic field, and this field may be induced (as a horizontal orvertical field, relative to a charging surface or power transfer surfaceof the primary unit) in proximity to the primary coil 12. Thiselectromagnetic field is employed in the system 1 to transfer power toone or more secondary units requiring power 20 located in proximity tothe primary unit 10, and/or adversely to one or more foreign objects 30also located in such proximity. A piece of metal may be considered to besuch a foreign object. Such foreign objects (as mentioned above) may beconsidered to be substantial ‘parasitic loads’.

The primary unit 10 may have any suitable form, for example having aflat platform forming a power transfer surface on or in proximity towhich the or each secondary unit 20 can be placed. In one case, theelectromagnetic field may be distributed over a power transfer area ofthe surface, as described in GB-A-2388716, the entire contents of whichare incorporated herein by reference. It will be appreciated that thisform of primary unit may allow one or more secondary units 20 and one ormore foreign objects 30 to be simultaneously located in proximity to theprimary unit to receive power therefrom. It will be appreciated thatmany other forms of primary unit 10 may allow one or more secondaryunits 20 and one or more foreign objects 30 to be simultaneously locatedin proximity to the primary unit to receive power therefrom. Anotherpossible form for primary unit 10 is a shelf, on which the secondaryunit 20 can be placed to receive power. Such a form may be advantageousfor allowing parts of the secondary device to sit outside the magneticfield.

The secondary unit 20 is separable from the primary unit 10 and includesa secondary coil 22 which couples with the electromagnetic fieldgenerated by the primary unit 10 when the secondary unit 20 is inproximity to the primary unit 10. In this way, power can be transferredinductively from the primary unit 10 to the secondary unit 20 withoutrequiring direct electrically-conductive connections therebetween.

The primary coil 12 and the or each secondary coil 22 may have anysuitable forms, but may for example be copper wire wound around ahigh-permeability former, such as ferrite or amorphous metal. Litz wireis a special type of wire which may be used in these circumstances. Litzwire has many strands of wire twisted together and can help reduce skinand proximity effects. The primary and secondary coils 12, 22 may bedifferent from one another, for example in size, number of turns, typeof core, and physical layout etc. Multiple primary and secondary coilsmay be employed. The number of primary and secondary coils may bedifferent from one another.

The secondary unit 20 may be connected to an external load (notshown—also referred to herein as the “actual load” of the secondaryunit), and may be configured to supply inductively-received power to theexternal load. The secondary unit 20 may be carried in or by an objectrequiring power (secondary device), such as a portable electrical orelectronic device or a rechargeable battery or cell. Further informationregarding possible designs of secondary unit 20 and the objects(secondary devices) that can be powered by the secondary unit 20 can befound in GB-A-2388716 (referred to above). In GB-A-2388716, suchsecondary units may be referred to as secondary devices.

In the context of the present invention, secondary units (and/orsecondary devices including such units) may be considered to be anyelectrical or electronic devices which require power, and may beportable such devices, for example (i.e. not exclusively) mobile phones,PDAs (Personal Digital Assistants), laptop computers, personal stereoequipment, MP3 players and the like, wireless headsets, vehicle chargingunits, home appliances such as kitchen appliances, personal cards suchas credit cards, and wireless tags useful for tracking merchandise.

In use, the primary unit 10 may enter a measurement phase, during whichthe adjustment unit 16 acts to drive the unit so that the magnitude ofan electrical drive signal supplied to one or more primary coils of theprimary unit changes, e.g. from a first value (characterising thesignal) to a second value (characterising the signal). Such driving maybe considered to change the amount of power that would be drawn from theprimary unit by a predetermined purely-resistive load, or an unregulatedload, or a test unit including substantially only a purely-resistiveload, in proximity thereto. The assessment unit 17 assesses the effectof such a change on a level of power being drawn from the primary unit.Such power may, in general terms, be drawn by a secondary unit 20 and/ora foreign object 30, although losses in so-called “friendly” parasiticloads (as discussed further below) may also need to be taken intoaccount. The detection unit 18 may detect the presence of a secondaryunit 20 and/or a foreign object 30 in proximity to the primary unitbased upon the assessment made in the assessment unit 17.

If a foreign object 30 is detected, the primary unit 10 may enter ashutdown mode. If no such foreign object 30 is detected, the primaryunit 10 may enter (or remain in) a charging mode if a secondary unit 20requiring power is detected, or a standby mode if no such secondary unit20 requiring power is detected. The standby mode may be entered forexample if a secondary unit 20 is present but does not require power.

Embodiments of the present invention may be considered to operate basedupon the following concept. In one embodiment, the secondary units 20are configured to have a known power-requirement characteristic inresponse to the change effected by the adjustment unit 16 of the primaryunit 10. In another embodiment, the secondary units 20 are regulated todraw a substantially constant amount of power (this being a preferredsuch power-requirement characteristic) from the primary unit 10 despitethe change effected by the adjustment unit 16. In contrast, a foreignobject is generally an unregulated load and the power drawn by theforeign object 30 may therefore change in correspondence with the changeeffected by the adjustment unit 16. So long as the power-requirementcharacteristic of the secondary unit 20 (i.e. to have substantiallyconstant power drawn, or some other such characteristic) issubstantially different from that of the foreign object, the primaryunit 10 may detect the presence of secondary units 20 and/or foreignobjects 30 in the detection unit 18 by assessing the power drawn fromthe primary unit in the assessment unit 17.

FIG. 2 is a flow chart representing a method 40 according to oneembodiment of the present invention. Method 40 may be carried out withinthe primary unit 10. Method 40 includes steps S2, S4 and S6.

Steps S2 and S4 are effectively carried out at the same time, orgenerally in parallel. In step S2, the magnitude of an electrical drivesignal supplied to one or more primary coils of the primary unit 10 ischanged. This may be carried out by the adjustment/control unit 16. Instep S4, the effect of the change on the power drawn from the primaryunit is assessed. This may be carried out by the assessment unit 17.

Step S6 is carried out after steps S2 and S4. In step S6, the presenceof a secondary unit 20 and/or foreign object 30 is detected based on theassessed effect determined in step S4. This may be carried out by thedetection unit 18.

Although not shown in FIG. 2, following the detection of step S6 theprimary unit may be placed into the charging, standby or shutdown modeof operation.

As mentioned above, the primary unit 10 may take the form of a flatpanel or other form enabling, for example, multiple secondary units 20to receive power therefrom simultaneously. Such a form could enable asingle secondary unit 20 to receive power from a number of differentpositions or orientations relative to the primary unit 10. The reader isdirected to GB-A-2388716 for an example of how to build such a form ofprimary unit. FIG. 3 is a schematic diagram of system 1 as seen fromabove, indicating this possibility. Primary unit 10 has three secondaryunits 20 (shown incorporated in portable electrical/electronic devices)resting on its power transfer surface for receiving power inductivelytherefrom. The three secondary units 20 are shown being of differenttypes/kinds (and/or incorporated in devices of different types/kinds),but they may be the same as one another. The primary unit 10 also has aforeign object 30 resting on its power transfer surface, which could bea metal object such as a set of keys. In this case, the detection of theforeign object 30 may cause the primary unit to enter the shutdown mode.

In the standby and shutdown modes, the supply of power by induction fromthe primary unit may be restricted or stopped to save power and/orprevent a foreign object from heating up. The primary unit may remain inshutdown mode until it is reset in some way. Such a reset could bemanually initiated by a user of the primary unit 10, or alternativelythe control unit 15 could periodically, or from time to time, start tosupply inductive power again and carry out a measurement phase byrepeating the steps of method 40 of FIG. 2. That is, from time to time,measurement phases may be carried out to determine whether the foreignobject 30 has been removed from proximity to the primary unit 10. Thesemeasurement phases may also detect whether secondary units 20 requiringpower are present or not.

FIG. 4 is a schematic diagram illustrating different modes of operationin system 1 and the conditions for switching between these differentmodes. The three modes of operation shown are an operating mode (or acharging mode) 60, a shutdown mode 62 and a standby mode 64. It will beappreciated that in one embodiment other modes of operation may exist,such as a “configuration” mode.

In the operating mode 60, primary unit 10 is in the charging state (i.e.supplying inductive power) most of the time, but periodically enters ameasurement phase 66, 68 as described above. If the result of themeasurement phase 66 is that it is determined that no secondary unit 20requires power, the primary unit 10 goes into the standby mode 64. Ifthe result of the measurement phase 68 is that it is determined that asignificant parasitic load (i.e. a foreign object 30) is present, theprimary unit 10 goes into the shutdown mode 62.

In the standby mode 64, the electrical drive unit 14 is stopped for mostof the time, thus consuming little power. Periodically, or from time totime, the primary unit 10 enters the charging state (i.e. to supplypower inductively) and carries out a measurement phase 70, 72 to checkwhether it should enter either the operating mode 60 or the shutdownmode 62. Otherwise, it remains in the standby mode 64.

The shutdown mode is functionally identical to the standby mode, withcorresponding measurement phases 74, 76. However, the two modes may bedistinguished by some user-interface features such as an LED to promptthe user to remove any substantial parasitic load (i.e. a foreign object30).

FIGS. 5(A) to 5(E) illustrate the conditions under which the modes ofFIG. 4 are selected in system 1. In FIG. 5(A) there is no secondary unit20 present in the vicinity of the primary unit 10. In this case, theprimary unit 10 is in the standby mode 64. In FIG. 5(B), no secondaryunit 20 is present but a substantial parasitic load (i.e. a foreignobject 30) is present in the vicinity of the primary unit 10. In thiscase, the primary unit 10 is in the shutdown mode 62. In FIG. 5(C) asecondary unit 20 and a substantially parasitic load are both present atthe same time in the vicinity of the primary unit 10. In this case, theprimary unit is in the shutdown mode 62. In FIG. 5(D), a secondary unit20 is present in the vicinity of the primary unit 10, but the load (theactual load) connected to the secondary unit 20 does not require anypower at the current time. In this case, the primary unit 10 is in thestandby mode 64. Finally, in FIG. 5(E), a secondary unit 20 is presentand its load requires power to charge or operate. Thus, the primary unit10 is in the operating mode 60.

FIG. 6 is a schematic diagram of a system 100 according to oneembodiment of the present invention. System 100 may be considered to beequivalent to system 1 of FIG. 1, and accordingly includes a primaryunit 10 having a primary coil 12, and a secondary unit 20 having asecondary coil 22. Power is transferred by electromagnetic inductionfrom the primary coil 12 to the secondary coil 22 in substantially thesame way as explained above in reference to system 1.

Although not shown in FIG. 6, it will be appreciated that system 100 mayinclude a plurality of secondary units 20 and that those secondary units20 may receive inductive power simultaneously from the primary unit 10.Furthermore, a foreign object 30 (also not shown in FIG. 6) may bepresent at the same time as such secondary units 20.

Primary unit 10 of system 100 includes, in addition to the primary coil12, a DC/DC converter 102, an inverter 104, a capacitor 106, a resistor108, a differential amplifier 110, buffers 112 and 114, and amicroprocessor unit (MPU) 116. The secondary unit 20 of system 100includes, in addition to the secondary coil 22, a rectifier 118, a DC/DCconverter 120, a load 122, a capacitor 124 and a differential amplifier126. Buffer 114 may be considered to be a peak detector, and is employedto measure the peak voltage over the coil 12.

It will be appreciated from FIG. 6 that the secondary unit 20 is shownincorporated within a portable device, being an object requiring power.For simplicity, the portable device is shown as being the same as thesecondary unit 20, however the secondary unit 20 may be a component (forexample removable) part of the portable device. Load 122 may thereforebe considered to be the actual load of the secondary unit 20, althoughit could be separate from the secondary unit 20. The primary unit 10 ofsystem 100 is shown as being a charger, operable to charge the portabledevice 20 by electromagnetic induction.

Within the primary unit 10 of the system 100, the DC/DC converter 102 isconnected to receive an external DC input, and is operable todown-convert the received DC input to a lower DC voltage V_(d). TheDC/DC converter 102 may be a switch-mode buck converter for highefficiency. The DC/DC converter 102 is connected to drive the inverter104, which generates an AC voltage at its output. The inverter 104 maybe a MOSFET half-bridge, driven from a reference oscillator (not shown).

The AC voltage output by the inverter 104 is used to drive the primaryinductive coil 12. The capacitor 106 is connected in series with theprimary coil 12, and the coil/capacitor combination is configured suchthat it is resonant at the operating frequency of the inverter 104. Inorder to reduce the harmonics present in the electrical drive signalsdriving the primary coil (i.e. the output of the inverter 104), it maybe desirable to provide an LC ballast circuit (not shown) between theinverter 104 and the primary coil 12. The peak coil voltage of theprimary coil 12, V_(pc), is typically much larger than the DC voltageV_(d) because the circuitry following the inverter (i.e. includingprimary coil 12 and capacitor 106) is configured to be resonant.

In the present embodiment, the operating frequency is consideredconstant and is thus not further commented upon. However, the operatingfrequency could of course be variable (i.e. tuneable) for efficiencyreasons. Indeed, the frequency could be tuned as a way of regulating thecoil voltage (i.e. the magnitude of the electrical drive signals in thecoil). For example, if the primary coil is configured to be resonant,then it is possible to vary the magnitude of the drive signals byvarying the frequency.

In the secondary unit 20 (portable device) of system 100, the secondarycoil 22 is connected to the input of the rectifier 118 in series withcapacitor 124, again such that the coil/capacitor combination isresonant. In use, the secondary coil 22 presents the rectifier with anAC voltage received via electromagnetic induction from the primary coil12. The rectifier 118 rectifies this AC voltage to output a DC voltageto the DC/DC converter 120. The DC/DC converter 120 down-converts therectified voltage from the coil to match the input voltage required bythe load 122.

DC/DC converter 120 may be a switch-mode converter (similarly toconverter 102) rather than a linear converter. A switch-mode converteris generally able to convert from one DC voltage to another DC voltagefar more efficiently than a linear converter. Furthermore, there istypically less variation in efficiency with input voltage for aswitch-mode converter than for a linear converter. A linear converterdrops any excess voltage across a resistance. Therefore, the larger thedifference between the input and output voltages, the lower theefficiency. This variation in efficiency with input voltage can renderthe power drawn by the secondary unit 20 of the system 100 notindependent of input voltage, which may make implementation of thepresent invention more difficult.

The DC/DC converter 120 of the secondary unit 20 is, optionally,configured to deliver a constant voltage to the load 122. This constantvoltage is maintained by means of a feedback loop including thedifferential amplifier 126. Essentially, the output voltage of the DC/DCconverter 120 is used to control the duty cycle of the DC/DC converter120 in order to maintain the required input voltage, V_(load), of theload 122 irrespective of changes to the input voltage of the DC/DCconverter 120.

Over time, the voltage requirements of the load 122 may change, e.g. ifthe load 122 is a battery having a charging cycle. The DC/DC converter120 may therefore be configured to maintain the required load voltageV_(load) at different levels for the different parts of such a chargingcycle. However, the required load voltage V_(load) typically changes ona relatively slow timescale, such that over a particular measurementphase or set of measurement phases it appears to be relatively constant.

The primary unit 10 of system 100 regulates the primary coil voltageV_(pc) at a predetermined voltage level. This is achieved by means of afeedback loop including the buffer (peak detector) 114 and themicroprocessor unit 116. As shown in FIG. 6, the primary coil voltage isessentially buffered by buffer 114 and input to the microprocessor unit116. Based upon the primary coil voltage, the microprocessor unit 116may control the duty cycle of the DC/DC converter 102 in order tomaintain the predetermined level of V_(pc). The feedback may be acombination of analogue feedback for fast response and digital feedbackvia the microprocessor unit 116 for large dynamic range. The primaryunit 10 is configured to maintain this predetermined primary coilvoltage V_(pc) irrespective of the load presented by the secondary unit20 (and/or any other such secondary units 20 or foreign objects 30).

The primary unit 10 of the system 100 is also able to determine theamount of power drawn via the primary coil 12. In this embodiment, thisis achieved by measuring both the voltage V_(d) and the current drawnfrom the DC/DC converter 102, I_(d). The voltage level V_(d) is input tothe microprocessor unit 116 via buffer 112, providing appropriatelevel-shifting and buffering. Resistor 108 is connected between theDC/DC converter 102 and the inverter 104 such that the current I_(d)passes therethrough. This current I_(d) is therefore measured bymeasuring the voltage across the resistor 108 with differentialamplifier 110, and the output of the differential amplifier 110 is inputto the microprocessor unit 116. Measuring the voltage and current atthis point has the advantage that the signals are DC. Within themicroprocessor unit 116, the voltages are sampled usinganalogue-to-digital converters (ADCs) and low-pass filtered to reducenoise. Averaging may be used as part of this filtering. The values ofthe voltage V_(d) and the current I_(d) are in the present embodimentdetermined within the microprocessor unit 116 and are multipliedtogether to determine the power drawn.

As mentioned above, when a metal object (i.e. a foreign object 30) iscoupled to the electromagnetic field induced by the primary coil 12,electric eddy currents are induced in the surface of the metal. As theseeddy currents are confined to the metal surface (determined by the skindepth), they have a reduced cross-section in which to circulate andtherefore can be subjected to a relatively high AC resistance. The metalobject therefore appears as a resistive load, the value of theresistance being dependent on the type of material, the geometry and thefrequency of operation (i.e. the frequency of the AC current passingthrough the primary coil 12).

FIG. 7 shows equivalent circuits of the primary coil 12 and resonantcapacitor 106, to show the effect that a foreign body 30 may have on theequivalent circuit seen by the primary coil 12.

FIG. 7( a) shows the primary circuit with the primary coil 12 andresonant capacitor 106, with a foreign body 30 in close proximity. Theforeign body is considered to be present in each of FIGS. 7( b) to 7(d)too, but is not shown for simplicity. In FIG. 7( a), the foreign body isconsidered to have no effect, i.e. as if it was absent. Accordingly, theequivalent circuit of FIG. 7( a) is the same as that in FIG. 6.

In a practical circuit, the inductor 12 and capacitor 106 will haveparasitics, such that they are not a pure capacitance and inductance(e.g. capacitor electrical series resistance, inductor resistance andinterwinding capacitance etc.).

FIG. 7( b) shows the electrical equivalent circuit when the foreign body30 is a conductor (e.g. copper or aluminium). Circulating eddy currentsare induced in the body 30. These act to reduce the inductance. Themetal will also have an AC resistance, depending on the thickness of theconductor, its resistivity and the frequency of the magnetic field. Thiswill result in additional loss. The effect is as if a series combinationof an inductance 132 and a resistance 134 were in parallel with theprimary coil 12. A thick piece of copper as the foreign body 30 wouldhave the dominant effect, for example at relatively low frequencies, ofan inductance change. However, a thin piece of copper as the foreignbody 30 would have the dominant effect of a resistance change.Generally, the effect of a conductor is to reduce the inductance andincrease the resonant frequency of the primary circuit. The losses willresult in power being dissipated, and the foreign body 30 heating up.Very high temperatures can be attained, particularly if the body 30 isnot that large and therefore unable to dissipate the heat effectively.Such metal present may therefore be seen as an increase in the powerdrawn from the supply.

FIG. 7( c) shows the electrical equivalent circuit when a nonconductiveor low conductive magnetic material is present (e.g. ferrite) as theforeign body 30. The presence of the magnetic material changes thereluctance of the overall magnetic circuit. This has the effect ofincreasing the effective inductance. However, losses will be introduced,which can be represented by a series resistance. The effect is as if aseries combination of an inductance 132 and a resistance 134 were inseries with the primary coil 12. Thus, the presence of a magneticmaterial will increase the inductance and lower the resonant frequencyof the primary circuit. The resistance 134 present will introducelosses, which in turn will increase the power drawn from the supply.

FIG. 7( d) shows the electrical equivalent circuit when there is aforeign body 30 present having both magnetic and conductive properties(for example silicon steel). The effect is as if a series combination ofan inductance 132 and a resistance 134 were in parallel with the primarycoil 12, and another such series combination were in series with theprimary coil. Such a material may increase or decrease the inductance,depending upon its composition. Or alternatively, the inductance may bepractically unchanged if the two inductance changes are similar. Thus,the resonant frequency may decrease, increase or remain the same.However, the resistance 134 present will introduce losses, which in turnwill increase the power drawn from the supply.

Thus, examining a change in resonant frequency may not be a reliableindication of the presence of foreign objects. A view may be taken thatmagnetic materials as foreign bodies are unlikely to come into contactwith the system. However, there is still the possibility that aconductive foreign body may be present at the same time as a legitimatesecondary device. The legitimate secondary device would typically haveeither a coil wound around a magnetic core, or alternatively a planarcoil with a magnetic shield behind it. The presence of the magneticmaterial associated with the device may increase the inductance, whilstthe presence of the conductive foreign body may lower the inductance.Depending on the relative magnitudes there could be an increase,decrease or no change in inductance, but typically the inductance changeof the legitimate device would dominate. It may be practically verydifficult to isolate the effect of the foreign body to reliably detectit in this way.

FIG. 8 shows a set of three graphs respectively showing waveforms forthe voltage V_(d), the current I_(d), and the power drawn P from theprimary unit 10. The waveform for the power drawn P is obtained bymultiplying the waveforms for the voltage V_(d) and the current I_(d).For FIG. 8, it is assumed that a secondary unit 20 requiring power is inproximity to the primary unit 10, and that no foreign objects 30 arepresent.

For the majority of the time, the primary unit 10 is in a “normal”state, and provides a constant voltage V_(d) to the input of theinverter 104. Periodically, or from time to time, a measurement phase iscarried out. During this phase, the voltage level V_(d) is changed, inthis case by increasing it by around 10%. The state in which the voltageV_(d) has been increased by 10% will be referred to as the “boost”state, and is identified as such in FIG. 8. It may be appreciated fromFIG. 8 that a second change occurs as the voltage level V_(d) returnsfrom the boost state to the normal state. Accordingly, it could beconsidered that two measurement phases have occurred, however for thepresent purposes concentration will be placed on the first change, i.e.from the normal state to the boost state.

This measurement phase is used to check for the presence of foreignobjects 30. In response to the increase in the voltage level V_(d)during the boost state, the AC coil voltage V_(pc) may also be boosted.As a result, the AC coil voltage in the secondary unit 20 will alsoincrease and in turn the rectified DC voltage output by the rectifier118 may increase too. However, as explained above, the DC/DC converter120 in the secondary unit 20 may adapt via the feedback loop so as tocontinue to provide a constant voltage V_(load) at the load 122. This inturn may mean that less current is drawn by the DC/DC converter 120 suchthat the total power drawn by the secondary coil 122 is approximatelyconstant (neglecting relatively insignificant changes in efficiency withinput voltage by both the rectifier 118 and the DC/DC converter 120).Accordingly, less current may be drawn by the primary coil 12 andtherefore the current I_(d) may also be reduced as shown in FIG. 8( b).Therefore, despite the voltage V_(d) increasing in the boost state,there may be a corresponding reduction in the current I_(d) such thatthe power drawn P from the DC/DC converter 102 in the primary unit 10remains approximately constant, as shown in FIG. 8( c).

FIG. 9 is similar to FIG. 8, except that its graphs represent the casewhen there is a metal object (foreign object 30) in proximity to theprimary coil 12, and in which no secondary units 20 requiring power arepresent. The primary unit 10 boosts the voltage level V_(d) in the sameway as in FIG. 8, i.e. from the normal state to the boost state.However, in the present case, instead of there being a regulated loaddrawing constant power, there is a resistive load present equivalent tothose shown in FIGS. 7( b) to 7(d). Such a resistive load has noregulation, and accordingly the current I_(d) therefore also increasesby approximately 10% as shown in FIG. 9( b) when the voltage V_(d)increases, and accordingly the power drawn P increases by approximately21%.

In a practical system there may be some non-linearity and this may needto be taken into account. For example, if diodes are employed in therectifier 118 then the voltage dropped over those diodes will generallynot change significantly, such that the efficiency of the rectifierincreases at the higher voltage (i.e. at the boosted voltage). Inaddition, the efficiency of the DC/DC converter 120 in reality willchange with the changing input voltage. It is therefore desirable tocompare the power change in FIG. 9( c) to a threshold level, and requirethe power difference to be above this threshold to establish that aforeign object is present (assuming that a power difference below thethreshold could be caused by such non-linearity).

FIGS. 10 and 11 are similar to FIGS. 8 and 9, except that it is nowassumed that a secondary unit 20 requiring power and a piece of metal(foreign object 30) are both present at the same time. In thissituation, the regulated load of the secondary unit 20 will result in areduction in the current I_(d) when the voltage V_(d) is increased, butthe metal (foreign object 30) will result in an increase in the currentI_(d) when the voltage V_(d) is increased. FIG. 10 shows the case wherethe current change due to the secondary unit 20 is greater than that dueto the metal, and FIG. 6 shows the case where the current change due tothe metal is greater than that due to the secondary unit 20. It will beappreciated that because a piece of metal constitutes a resistive loadwithout regulation, the power drawn P should increase if metal ispresent.

Based on the above, it will be appreciated that by assessing the changein the power drawn P when the voltage level V_(d) is changed, it ispossible to determine whether a secondary unit 20 requiring power and/ora foreign object 30 is present in proximity to the primary unit 10. Oneexample way of carrying out this assessment is to take a measurementbefore and after that change. FIG. 12 is an enlarged version of FIG. 8(a), intended to provide an example as to when such measurements could betaken. The power drawn P may be first measured in the normal state.Sufficient time S1 is allowed for the system to settle from any otherevents that may have occurred. During this time 51, the filters may bereset. The current I_(d) and voltage V_(d) may then be sampled over ameasurement time interval, A, during the normal state, and an average orfiltered value taken. As mentioned above, the power drawn P iscalculated by multiplying the voltage V_(d) and current I_(d) valuestogether. Next, the voltage level V_(d) is increased by 10% into theboost state. After another settling period S2 from the previousmeasurements, the currents and voltages are again sampled andaveraged/filtered during a second measurement time interval B. Again,the power drawn P in the boost state is calculated by multiplying thevoltage and current values together.

Technically, it is possible in one embodiment just to measure thecurrent (as the voltage is being controlled) to assess the power level.That is, it is to some extent redundant to measure the voltage andmultiply the voltage by the current. In a simple form, the invention maybe embodied by setting one voltage and measuring the current and thensetting another voltage and measuring the current. That is, it is notessential to obtain a power value specifically, rather just a“measurement” or “indicator” of power level. The embodiments disclosedherein should therefore be interpreted accordingly.

If the power drawn P in the boost state exceeds that drawn in the normalstate by a predetermined amount (for example, exceeding a thresholdamount), then it may be determined that a foreign object is present inproximity to the primary unit. However, in one embodiment it is possiblefor the load in a secondary unit to change between the two measurements,for example if the charging cycle of a load 122 such as a batterychanges from one zone to the next. It is therefore desirable to carryout two sets of measurements in succession. These sets may be consideredto include different measurement phases or may be considered together toform a single measurement phase. If both sets of measurements areconsistent, then it may be determined that a foreign object 30 is indeedpresent.

FIG. 13 shows timing diagrams for the system 100 under differentconditions. FIG. 13( a) shows the system 100 in normal operation whenthere is a secondary unit 20 in proximity to the primary unit 10 and noforeign object 30 present. Periodically, or from time to time, theprimary unit 10 checks to see if any foreign objects 30 are present byboosting the voltage level V_(d) and making a set of two measurements,as described above. If the result of the two measurements is within acertain tolerance, then the system may deduce that no foreign object 30is present and that it may not be necessary to take another set ofmeasurements. The system then waits for a predetermined period beforecarrying out another set of such measurements. In the example shown inFIG. 13( a), there is an example period of 500 milliseconds between eachset of such measurements, and the boost state is maintained for exampleperiods of 10 milliseconds.

FIG. 13( b) shows the system when a foreign object 30 is detected. Here,the first measurement set is assumed to show a significant difference inpower levels, so it is immediately followed by a second set of suchmeasurements. If the two sets of measurements are consistent with oneanother, the system then reduces the power supply to zero, to preventpower from being delivered into the foreign object 30 and heating it.This may be considered to be equivalent to the system entering theshutdown mode as discussed above.

FIG. 13( c) shows the system when a foreign object 30 is present. It istherefore assumed that the system is in the shutdown mode, and that fromtime to time the system checks to see if the foreign object 30 has beenremoved. Accordingly, during most of the time, there is no voltagepresent (i.e. the voltage level V_(d) is zero), in order to prevent theforeign object from heating up. Periodically, the voltage level V_(d) israised to the normal state before the measurements can be taken. If theforeign object 30 is still present, then the primary unit 10 will taketwo sets of measurements and then reduce the voltage level V_(d) to zeroagain. However, if the foreign object 30 has been removed then the firstset of power measurements will be substantially the same as one anotherand the normal operating state can be resumed. This may be considered tobe equivalent to the system leaving the shutdown mode and entering theoperating mode of operation.

It is possible in certain cases that the secondary-device loadregulation will function if lower voltages (than the normal andboost-state voltages) are applied. In this case it may be possible tomake measurements at two voltage levels which are both below (or one ofwhich is below) that of the normal and boost states.

FIG. 14 is a flow diagram of a method 200 according to one embodiment ofthe present invention. Method 200 includes steps S200 to S244, and maybe employed by system 100.

In step S200, the power (i.e. the voltage level V_(d)) is set to the‘normal’ state, and the system is then allowed to settle in step S202,in this case for 10 ms. In step S204, the power drawn P is measured andstored as a value P1 in step S206. In step S208, a check is made to seeif the power drawn, P1, is greater than a certain threshold power level,X. If the power drawn P1 is less than or equal to X, then it is assumedthat there is no device requiring power and the power is turned off(i.e. the voltage level V_(d) is set to zero) in step S210. If there areno secondary units requiring power, then the system will wait for apredetermined amount of time, in this case for 500 ms, in step S212before returning to step S200 to see if a secondary unit has appeared.It may be that there is a secondary unit present (for exampleincorporated in a secondary device), but that the secondary unit doesnot require power, as considered below.

If in step S208 it is determined that the power drawn P1 is greater thanX, then the power (i.e. the voltage level V_(d)) is raised to the‘Boost’ state in step S214. The system is then allowed to settle in stepS216, again for 10 ms. In step S218, the power drawn P is measured andstored as a value P2 in step S220. The power is then returned to the‘Normal’ state in step S222.

In step S224, it is determined whether the difference between P2 and P1,i.e. P2-P1, is less than or equal to a given threshold, Y. If this isthe case, the values P2 and P1 are considered to be substantially thesame as one another and it is therefore assumed that there is no metal(i.e. no foreign object 30) present. In this case, the method leaves thepower in the ‘Normal’ state and proceeds to step S212 in which thesystem will wait for a predetermined amount of time, in this case for500 ms, before returning to step S200.

If P2-P1 is determined to be greater then the threshold Y, it may meanthat there is metal (i.e. a foreign object 30) present, or it could meanthat the actual load of a present secondary unit 20 has changed betweenthe two measurements. In order to resolve this ambiguity, two moremeasurements are taken for each of the ‘Normal’ and ‘Boost’ states.

In this regard, the method proceeds to step S226 in which the system isallowed to settle in the ‘Normal’ state, in this case for 10 ms. In stepS228, the power drawn P is measured and stored as a value P3 in stepS230. In step S232, the power (i.e. the voltage level V_(d)) is thenraised to the ‘Boost’ state, and the system is then allowed to settle instep S234, again for 10 ms. In step S236, the power drawn P is measuredand stored as a value P4 in step S238. The power is then returned to the‘Normal’ state in step S240.

In step S242, it is determined whether the difference between P4 and P3,i.e. P4-P3, is less than or equal to threshold, Y. This is similar tothe determination made in step S224, and accordingly it will beappreciated that in this way the second set of measurements (P3, P4) canbe compared the first set of measurements (P1, P2).

If the second set of measurements also indicates a difference greaterthan Y, then the system determines that metal (i.e. a foreign object 30)is present and sets the power to ‘Off’ by proceeding to step S210.Otherwise, the method leaves the power in the ‘Normal’ state andproceeds to step S212 in which the system will wait for a predeterminedamount of time, in this case for 500 ms, before returning to step S200.By returning to step S200, the system checks again to see if there hasbeen any change (e.g. some metal placed in proximity to the chargercoil).

The threshold level Y needs to be large enough to accommodate any errordue to noise present and any uncertainty in the system. It may bepossible to reduce the uncertainty which will allow lower levels ofparasitic loss to be detected.

The losses in the system may be apportioned as:

1. Fixed pad losses (i.e. in the primary unit 10)

2. Variable pad losses (i.e. in the primary unit 10)

3. Fixed receiver losses (i.e. in the secondary unit 20, or in thesecondary device)

4. Variable receiver losses (i.e. in the secondary unit 20, or in thesecondary device)

5. Load (i.e. in the actual load 122)

6. ‘Parasitic’ losses (e.g. in foreign metal objects)

7. ‘Friendly Parasitic’ losses (e.g. in metal present within thesecondary unit or device)

The fixed losses (1,3) should remain the same between measurements,irrespective of being in the ‘Normal’ or ‘Boost’ states. The load shouldalso remain the same between measurements (a second set of measurementsis used to cope with the case that the load changes significantlybetween measurements). The variable losses (2,4) add measurementuncertainty. It may be possible to compensate for this uncertainty bycalibrating the system for efficiency against coil voltage. Theresulting measurement will detect a combination of ‘Parasitic’ and‘Friendly Parasitic’ losses (6,7). It may be possible to determine the‘Friendly Parasitics’ (7) to reduce the uncertainty. For instance, theportable device (secondary unit 20 or secondary device incorporating thesecondary unit 20) may communicate what its ‘Friendly Parasitics’ are tothe charger (primary unit 10) on start-up (or it may communicate a codesuch as a device type which represents this information). Accordingly,by employing such additional information it may be possible to improvethe accuracy and robustness of the system.

FIG. 15 is a schematic diagram of a system 300 according to oneembodiment of the present invention. System 300 may be considered to beequivalent to systems 1 and 100, and accordingly includes a primary unit10 having a primary coil 12, and a secondary unit 20 having a secondarycoil 22. Power is transferred by electromagnetic induction from theprimary coil 12 to the secondary coil 22 in substantially the same wayas explained above in reference to system 1.

Although not shown in FIG. 15, it will be appreciated that system 300may include a plurality of secondary units 20 and that those secondaryunits 20 may receive inductive power simultaneously from the primaryunit 10. Furthermore, a foreign object 30 (also not shown in FIG. 6) maybe present at the same time as such secondary units 20.

System 300 is closely similar to system 100, and accordingly the samereference numerals are employed for simplicity and duplicate descriptionis omitted. System 300 differs from system 100 in that the power drawn Pis determined by measuring the voltage and current at the primary coil12. This has the advantage that the measurement is more accurate becauseit is directly at the primary coil 12, rather than at the input to theinverter 104 as in system 100. However, system 300 may be lessadvantageous than system 100 because i) the voltage and current measuredat the primary coil 12 are AC and therefore may be harder to determine,particularly if the waveforms are distorted; ii) it may be desirable todetermine the phase angle between the voltage and current to establishpower drawn as distinct from energy stored in the resonant circuitincluding the primary coil 12 and capacitor 106; iii) the voltage ismuch higher at the primary coil 12 than at the input to the inverter104.

In system 300, the voltage at the primary coil 12 is measured with apeak detector (implemented by buffer 114) and the current is measuredwith a current transformer/sensor 302 (via a buffer 312). Equally, thecurrent could be measured using a series sense resistor, as in system100. In order to determine the power drawn P by the coil 12, themicroprocessor unit 316 (equivalent to microprocessor unit 116 in system100) measures the rms AC voltage, V_(ac) (equivalent to voltage V_(pc))across the primary coil 12, measures the rms AC current, I_(ac), passingthrough the primary coil 12, and determines the phase difference φbetween them. The power drawn is then given by P=V_(ac)I_(ac)|cos φ|. Inmeasuring the AC current and voltage, it may be desirable to use alock-in amplifier (either digital within the microprocessor 316 oranalogue external to it). This may use the reference oscillator used inthe inverter 104 to ‘lock-in’ on the required signal and dramaticallyimprove the signal-to-noise ratio (SNR).

Looking at systems 100 and 300, it is possible in other embodiments tomeasure the voltage and current desirable for determining the powerdrawn P at various other points in those systems, for example at theinput to the DC/DC converter 102.

In the above embodiments, the main configuration considered is that ofsupplying a constant voltage to load 122. However, it may be possible inother embodiments to charge a battery directly. FIG. 16 shows a typicalcurrent and voltage profile for charging a Lithium Ion battery (theprofiles for Lithium Polymer batteries and other derivatives aresimilar). Initially, if the battery is significantly discharged,constant current is supplied at a low level (at around 10% of maximum),and this is commonly referred to as trickle charging. This continuesuntil the battery reaches around 3V. After this point, constant currentis supplied at the maximum level until the battery reaches 4.2V. At thispoint, the output voltage is regulated to 4.2V and the current graduallyreduces until the charging current is around 7% of maximum. Thus bothconstant current regulation and constant voltage regulation may beemployed at different points during the charge cycle.

FIG. 17 is a schematic diagram of a secondary unit 420 which may besubstituted for the secondary unit 20 in systems 1, 100 and 300 to formfurther embodiments of the present invention. Accordingly, thoseelements in secondary unit 420 already described above with reference tosecondary unit 20 are denoted by the same reference numerals andduplicate description is omitted.

Secondary unit 420 has a battery 422 as its actual load (i.e. in placeof load 122). A charge controller 424 is disposed between the rectifier118 and the battery 422, and includes a DC/DC converter 424,differential (or even operational) amplifiers 428, 430 and 432, and aresistor 434. The DC/DC converter 424 is connected between the rectifier118 and the battery 422 in a similar way to the DC/DC converter 120 insecondary unit 20, i.e. to down-convert the voltage output by rectifier118 for supply to the battery 422. The resistor 434 is connected betweenthe DC/DC converter 426 and the battery 422 such that the currentI_(load) flowing into the battery passes therethrough. Differentialamplifiers 428 and 430 are connected to measure this current as avoltage over the resistor 434 and input the measurement (current sense)of the current I_(load) into the DC/DC converter 426. Operationalamplifier 432 is connected to measure the voltage V_(load) supplied tothe battery 422, and to input this measurement (voltage sense) also tothe DC/DC converter 426. Looking back to FIG. 16, during the constantvoltage phase aim is to regulate the voltage, and during the constantcurrent phase the aim is to regulate the current.

The output from both paths of differential (or operational) amplifiers428, 430, 432 is used to control the duty-cycle of the DC/DC converter.Changing the duty cycle, changes the ratio of input voltage to outputvoltage. Equally, at a given instant in time, there will be some form ofload on the output, so the DC/DC voltage output can be adjusted to givea required current. The secondary unit 420 could be considered to have acontrol unit which takes the output from both paths as inputs, andadjusts the duty cycle of the DC/DC converter accordingly. The controlfunction may be embodied in many different ways, for example as anelectronic chip including appropriate logic circuitry controllingMOSFETs.

Incidentally, it is noted that the DC/DC converters in any of theembodiments of the present invention could be upconverters (Boostconverters) or up/down converters (Buck-Boost converters) instead ofdown converters. However, down-converters tend to be more efficient.

During the constant current phases, the current sense is primarily usedto regulate the current I_(load) to be constant. When the voltage V_(d)in the primary unit 10 is increased, there will be a correspondingincrease in the voltage over the receiver coil 22, and hence in therectified voltage input to the DC/DC converter 426. With no feedback,both V_(load) and I_(load) would increase. However, the feedback fromthe current sense acts to ensure that the load current, I_(load),remains constant. This results in the current in the receiver coil(secondary coil) 22 being reduced. Consequently, the current in theprimary coil 12 is reduced. Thus the power drawn P by the primary coilshould remain approximately the same, ignoring any change in efficiencywith voltage level or other variable losses.

In both voltage regulation and current regulation modes, the power drawnby the charger controller 424 may be approximately independent of thevoltage at the secondary coil 22. Any change in secondary coil voltagecan be reflected in a change in secondary coil current. The powerrequirement of the charge controller 424 will vary over time, but thiswill be relatively slow compared to the measurements made during use ofembodiments of the present invention described above. Accordingly, whenthe primary coil voltage is increased, there will be a correspondingdecrease in primary coil current, such that the total power drawn Premains approximately the same.

It has been found that some charge controllers 424 have repeating spikesof current occurring during the trickle charge phase. If the period ofthese spikes happens to coincide with the period at which themeasurements are made, then it can cause an erroneous result. This maybe combated by taking three sets of measurements instead of two, and byensuring that the time between the second and third set is sufficientlydifferent to the time between the first and second sets.

It will be appreciated that the DC/DC converters 120, 426 need notoperate under constant current or voltage conditions. The system willoperate properly if the power drawn P by the secondary unit does notchange (or changes predictably, i.e. in some predetermined manner) withinput voltage across the secondary coil 22. This is irrespective of whatvoltage or current is supplied to the load 122, 422.

FIGS. 18 and 19 are schematic diagrams of secondary units 520 and 620,respectively configured to enable the system (further including aprimary unit) to focus on regulation of the power drawn P, rather thanspecifically the voltage or current supplied to the load 122, 422.Secondary units 520 and 620 may be substituted for secondary units 20and 420 to form further embodiments of the present invention.

Referring to FIG. 18, the secondary unit 520 includes (in addition tothose elements denoted by the same reference numerals as in secondaryunit 20 of FIG. 6) an operational amplifier 502, a resistor 504, avoltage sense point 505 and a control unit 506. As will be appreciatedby comparison to FIG. 6, the operational amplifier 502, the resistor504, and the voltage sense point 505 are connected to provide voltageand current measurements to the control unit 506, representative of thevoltage and current input to DC/DC converter 120. The control unit 506is operable to adjust the operation of the DC/DC converter 120 to modifythe input current so that the overall power drawn P from the primaryunit remains constant when the input voltage changes.

Referring to FIG. 19, the secondary unit 620 includes (in addition tothose elements denoted by the same reference numerals as in secondaryunit 20 of FIG. 6) an AC current sense point 602, a buffer 604, an ACvoltage sense point 605 and a control unit 606. As will be appreciatedby comparison to FIG. 15, the AC current sense point 602, buffer 604,and AC voltage sense point 605 are connected to provide AC voltage andcurrent measurements to the control unit 606, representative of the ACvoltage and current input to rectifier 118. The control unit 606 isoperable to monitor the AC coil voltage and adjust the operation of theDC/DC converter 120 such that the overall power drawn P is independentof the (primary or secondary) coil voltage.

FIG. 20 is a schematic diagram of a primary unit 710 according to oneembodiment of the present invention that is identical to primary unit 10of FIG. 6, except that an LC ballast circuit 702 is present between theinverter 104 and the primary coil 12 and capacitor 106. Accordingly,those elements common between primary units 710 and 10 are denoted bythe same reference numerals and duplicate description is omitted. LCballast circuit 702 includes a series inductor 704 and a parallelcapacitor 706, connected to form a low-pass filter. It is advantageousto provide the LC ballast circuit 702, as the low pass filtering has theresult of reducing the harmonics from the inverter 104. This may behelpful as unwanted harmonics can cause interference in other equipment(e.g. radio receivers) or prevent the system from complying withregulatory emissions.

FIGS. 21 to 23 are schematic diagrams of primary units 810, 910 and1010, respectively, each of which is closely similar to a primary unitdescribed above and therefore forms a further embodiment of the presentinvention. Accordingly, those elements of primary units 810, 910 and1010 already described above are denoted by the same reference numeralsand duplicate description is omitted.

The common feature between primary units 810, 910 and 1010 is that theyeach have multiple primary coils 12A, 12B, 12C . . . , rather than asingle primary coil 12. It is possible to employ multiple primary coilsso that multiple secondary units (or secondary devices incorporatingsuch secondary units) can be charged simultaneously. The primary coils12A, 12B, 12C . . . are configured to be in parallel with one another.Multiple primary coils may be present for reasons other than for copingwith multiple secondary units. For example, multiple primary coils maybe provided for redundancy reasons, or so that a single secondary devicecan receive power in any of a number of locations (defined by thedifferent primary coils) relative to the primary unit. Additionally,instead of effecting the change in power available by altering thevoltage (or the magnitude of some other signal) supplied to a soleprimary coil, it would be possible to switch from one primary coiloperated at a first voltage to a second primary coil operated at asecond voltage different from the first voltage. That is, the differencebetween “normal” and “boost” states may be effected by switching betweenprimary coils, or by changing the number of primary coils that areactive simultaneously, or changing the magnitudes of signals supplied tothe various primary coils, or some combination of these methods.

Methods embodying the present invention described above may be used inthe same manner when primary units 810, 910 and 1010 are employed, onedifference being that there is a parallel combination of primary coilspresent rather than a single such primary coil. It will be appreciatedthat primary unit 810 is similar to primary unit 10 of FIG. 6, in whichthe power drawn P is measured before the inverter 104, and that primaryunit 910 is similar to primary unit 10 of FIG. 15, in which the power ismeasured after the inverter 104.

In this arrangement, measurements may also be taken at the node beforethe resonant capacitor in order to reduce phase errors and improve powermeasurement accuracy. Rather than having a single current sense for allof the coils, it can be advantageous in one embodiment to have a currentsense on each individual coil, as shown in FIG. 23. This can make iteasier to deduce the total amount of power drawn, particularly wherethere are widely varying loads or in the case where some coils havedevices being powered and other coils do not. The different primarycoils may for example be distantly located from one another, althoughthey may be relatively closely located together, for example in anarray.

Incidentally, for AC current measurement, either a current transformer(a current sense), as in FIG. 22, or a sense resistor, as in FIG. 23,may be used. Either method can involve a peak detector or an averagerfor the measurement. One advantage of a current sense is that itintroduces less loss than a resistor. However, a current sense isgenerally more expensive than a resistor. If there is a large array ofcoils, as in FIG. 23, then it would probably be cost-effective to usesense resistors.

FIGS. 24 and 25 are schematic diagrams of possible coil layouts on thecharging surfaces of primary units of some embodiments of the presentinvention. In some such embodiments, a secondary unit may be placedanywhere, or substantially anywhere, on such charging surface to becharged. In each case shown, the primary unit concerned includes aplurality of primary coils. In FIG. 24, the charging surface has anarray of wound ferrite cores 1100, i.e. an array of wound coils 1100 ona ferrite back-plate 1102. In FIG. 25, the charging surface has an arrayof printed hexagonal spiral coils 1200 etched onto a PCB (PrintedCircuit Board) 1202, which may have a ferrite and/or metal shieldunderneath. In FIG. 25, each hexagonal arrangement 1200 may beconsidered to be an individual coil. Rectangles 1204 represent thepossible footprints of a secondary unit, or a secondary deviceincorporating such a secondary unit, placed on the charging surface ofthe primary unit concerned to be charged (i.e. to receive powerinductively therefrom). It will be appreciated that in one embodimentthe footprint of the secondary unit may be smaller than the chargingarea on the charging surface, such that multiple secondary units may becharged at the same time.

Primary units embodying the present invention and having multipleprimary coils 12A, 12B, 12C, . . . , may be configured to operate bysupplying current to those primary coils for which there is a secondarycoil 22 of a secondary unit in proximity thereto, and by supplying nocurrent to other primary coils (so as to conserve power). The methodsembodying the present invention described above may therefore beemployed in such primary units. That is, it may be considered that thereis an array of electrically-parallel primary coils, rather than just asingle coil. However, in this case the primary unit may be configuredsuch that each primary coil can be ‘switched’ in and out of circuit, sothat only the appropriate coils are activated. FIG. 26 is a schematicdiagram of such a primary unit 1310.

Primary unit 1310 is closely similar to primary unit 810 of FIG. 21described above. Accordingly, those elements of primary unit 1310already described above are denoted by the same reference numerals andduplicate description is omitted. Primary unit 810 includes switchesSW-A, SW-B, SW-C, . . . , connected in series with primary coils 12A,12B, 12C, . . . , respectively, and operable to switch their respectiveprimary coils in and out of circuit. It may be appreciated that byanalogy primary units equivalent to those shown in FIGS. 22 and 23 couldalso be implemented.

It may be desirable to ensure that the overall inductance remains thesame to keep the system on resonance. This can be achieved by havingseparate inductors in the primary unit to be used as ‘dummy coils’. Thusif fewer primary coils than the maximum are energised (switched intocircuit), then a required number of additional ‘dummy coils’ may beswitched into circuit to keep the overall inductance the same.

In some embodiments of the present invention, it is possible to employmany different types of DC/DC converter, including Buck Converters,Boost Converters and Buck-Boost Converters of many different topologies.It is possible for secondary units embodying the present invention toinclude loads (even multiple loads) that may be constant-current,constant-voltage loads, or some other combination of the two. Forinstance, a portable device (secondary device incorporating a secondaryunit) may need power for its internal functionality in addition to thatrequired by a charge controller to charge a battery.

In some embodiments of the present invention, it is not essential forthe regulation performed in the secondary unit to be constant-current orconstant-voltage. Systems embodying the present invention may operate ifthe power drawn P by the portable device does not change with inputvoltage (e.g. voltage V_(d) in the primary unit). This is irrespectiveof what voltage or current is supplied to the load in the secondaryunit. The regulation may not be constant-power either, and may beconfigured to have a known dependence of the power drawn P with respectto input voltage. In this situation, the primary unit (charger) may takepower measurements at two primary coil voltage levels (constituting ameasured power-requirement variation). Using knowledge of the couplingbetween the primary coil and secondary coil, the two correspondingsecondary-coil voltages may thus be determined. Using knowledge of thepower-requirement variation with input voltage of the secondary unit,the expected power-requirement variation for the two measurements maythus be determined. If the measured power-requirement variation issubstantially different from (e.g. greater than) the expectedpower-requirement variation, then the charger deduces that there must bemetal (i.e. a foreign object) present. It may be desirable for thereceiver (secondary unit) to communicate information to the charger(primary unit) relating to the degree of coupling between the chargerand receiver or to communicate the received voltage directly. It may bedesirable to perform more than two measurements and to fit a polynomial.Other derived information (e.g. the derivative) may be used indetermining whether there are foreign objects present.

Although the embodiments described above output DC voltages to a load,and consequently have a rectifier in the portable device (secondaryunit), this does not have to be the case. For instance, it is possibleto supply an AC voltage to the load. In this case, it would still bepossible to implement the present invention, i.e. to ensure that thepower drawn P by the secondary unit either remains constant or has aknown dependence on input voltage in the absence of foreign objects.

Some embodiments of the present invention are advantageous becausecommunications between the portable device (secondary unit) and thecharger (primary unit) are not essential, i.e. are optional. Someembodiments of the present invention may thus be lower in cost thansystems where communication is essential. Some embodiments of thepresent invention are able to detect metal (foreign objects) in thepresence of a valid portable device (secondary unit). Some embodimentsof the present invention are also not ‘fooled’ by steel or siliconsteel, i.e. they can differentiate between secondary units and suchforeign objects, because they do not rely on a phase/frequency shiftmeasurement. Some embodiments of the present invention may also becost-effective in terms of hardware as many aspects of the presentinvention can be implemented in software within a microprocessor.

In any of the above aspects of the present invention, particularly inthe method aspects, the various features may be implemented in hardware,or as software modules running on one or more processors. Features ofone aspect may be applied to any of the other aspects.

The invention also provides a computer program or a computer programproduct for carrying out any of the methods described herein, and acomputer readable medium having stored thereon a program for carryingout any of the methods described herein. A computer program embodyingthe invention may be stored on a computer-readable medium, or it could,for example, be in the form of a signal such as a downloadable datasignal provided from an Internet website, or it could be in any otherform.

The invention claimed is:
 1. A detection method for use in a primaryunit of an inductive power transfer system, the primary unit beingoperable to transmit power wirelessly by electromagnetic induction to atleast one secondary unit of the system located in proximity to theprimary unit and/or to a foreign object located in said proximity, saidat least one secondary including a regulator with an operatingthreshold, the method comprising: driving the primary unit to transmitpower sufficient to operate the regulator at or above the operatingthreshold and so that in a driven state a magnitude of an electricaldrive signal supplied to one or more primary coils of the primary unitchanges from a first value to a second value, wherein power transmittedat the first value is sufficient to operate the regulator at or abovethe operating threshold and power transmitted at the second value issufficient to operate the regulator at or above the operating threshold;assessing the effect of such driving on an electrical characteristic ofthe primary unit based on a difference between a first measurement ofthe electrical characteristic at the first value and a secondmeasurement of the electrical characteristic at the second value; anddetecting in dependence upon the assessed effect the presence of a saidsecondary unit including said regulator and/or a foreign object locatedin proximity to said primary unit.
 2. A detection method as claimed inclaim 1, wherein the electrical characteristic of the primary unit is alevel of power being drawn from the primary unit.
 3. A detection methodas claimed in claim 1, wherein said values characterize the electricaldrive signal.
 4. A detection method as claimed in any of the precedingclaim 1, wherein said values are peak values or root-mean-square valuesof an alternating potential difference supplied across one or moreprimary coils of the primary unit.
 5. A detection method as claimed inclaim 1, wherein said values are peak values or root-mean-square valuesof an alternating current passing through one or more primary coils ofthe primary unit.
 6. A detection method as claimed in claim 1,comprising maintaining said first and second values steady for longenough for operation of the primary unit to stabilise.
 7. A detectionmethod as claimed in claim 1, wherein the second value is larger thanthe first value.
 8. A detection method as claimed in claim 1, whereinthe primary unit comprises conversion means for converting a DCelectrical drive signal into a time-varying electrical drive signal forsupply to the one or more primary coils concerned, and wherein suchdriving comprises controlling operation of the conversion means.
 9. Adetection method as claimed in claim 7, wherein operation of saidconversion means is governed by a duty cycle, and wherein such drivingcomprises controlling the duty cycle of the conversion means.
 10. Adetection method as claimed in claim 1, wherein such driving comprisesreconfiguring operation of the primary unit from an existing statepreceding said change to a changed state succeeding said change, andwherein such assessment comprises obtaining a measurement of theelectrical characteristic of the primary unit in the existing state andin the changed state.
 11. A detection method as claimed in claim 10,comprising maintaining the first value during the existing state andmaintaining the second value during the changed state.
 12. A detectionmethod as claimed in claim 8, wherein such assessment comprises takingvoltage and/or current measurements in respect of primary-coil signals.13. A detection method as claimed in claim 12, further comprising takingsaid voltage and/or current measurements in respect of the DC electricaldrive signal.
 14. A detection method as claimed in claim 13, furthercomprising taking said voltage and/or current measurements in respect ofthe time-varying electrical drive signals.
 15. A detection method asclaimed in claim 14, comprising taking a series of samples of saidvoltages and/or currents, and basing such assessment on the series ofsamples.
 16. A detection method as claimed in claim 1, wherein saiddriving and assessing form a set of method steps, the method comprisingcarrying out a plurality of such sets and basing such detection on twoor more of such sets.
 17. A detection method as claimed in claim 1,comprising determining that a said foreign object is present inproximity to said primary unit if it is determined that the electricalcharacteristic of the primary unit has substantially changed as a resultof the driving the primary unit so that in a driven state the magnitudeof an electrical drive signal supplied to one or more primary coils ofthe primary unit changes from the first value to the second value.
 18. Adetection method as claimed in claim 1, wherein the or each saidsecondary unit of the system is configured such that, when in proximityto the primary unit and receiving power inductively therefrom, anelectrical characteristic of the secondary unit responds to such drivingin an expected manner, the method further comprising determining whethera said secondary unit and/or a foreign object is present in proximity tosaid primary unit in dependence upon results of such assessment and theor each such expected response.
 19. A detection method as claimed inclaim 18, wherein for the or each said secondary unit the electricalcharacteristic of that secondary unit is its power drawn from theprimary unit.
 20. A detection method as claimed in claim 19, comprisingdetermining that a foreign object is present if the results of theassessment at least partly do not correlate with the or any saidexpected response.
 21. A detection method as claimed in claim 20,comprising determining that a secondary unit is present if the resultsof the assessment at least partly do correlate with the or one saidexpected response.
 22. A detection method as claimed in claim 21,wherein, for the or at least one said secondary unit, the expectedresponse is that its electrical characteristic does not substantiallychange in response to such driving the primary unit so that in thedriven state the magnitude of an electrical drive signal supplied to oneor more primary coils of the primary unit changes from the first valueto the second value.
 23. A detection method as claimed in claim 22,wherein the expected response for one said secondary unit is differentfrom the expected response for another such secondary unit.
 24. Adetection method as claimed in claim 23, further comprising, in theprimary unit, receiving from the or each secondary unit that is in apower requiring state, information relating to said expected responsefor the secondary unit concerned.
 25. A detection method as claimed inclaim 24, wherein said information identifies the type of secondary unitconcerned, and wherein the method further comprises determining theexpected response based upon the identified type of secondary unitconcerned.
 26. A detection method as claimed in claim 1, furthercomprising employing, when carrying out said detection, secondary-unitcompensation information relating to a parasitic load imposed on theprimary unit by the or each secondary unit so as to compensate for saidparasitic load of the or each secondary unit.
 27. A detection method asclaimed in claim 26, comprising receiving, from the or each secondaryunit, such secondary-unit compensation information.
 28. A detectionmethod as claimed in claim 27, wherein part or all of said informationis received via a communication link separate from a link constituted bythe transfer of inductive power.
 29. A detection method as claimed inclaim 28, wherein said communication link is an RFID link.
 30. Adetection method as claimed in claim 29, wherein part or all of saidinformation is received via an inductive communication link constitutedby the transfer of inductive power.
 31. A detection method as claimed inclaim 1, further comprising employing, when carrying out such detection,primary unit compensation information relating to losses in the primaryunit itself so as to compensate for said losses.
 32. A detection methodas claimed in claim 31, further comprising deriving part or all of saidprimary-unit compensation information from measurements taken by theprimary unit when it is effectively in electromagnetic isolation.
 33. Adetection method as claimed in claim 1, further comprising, followingdetection of a foreign object in proximity to the primary unit,restricting or stopping inductive power supply from the primary unit.34. A detection method as claimed in claim 1, further comprising,following detection of one or more secondary units requiring power,maintaining or adjusting inductive power supply from the primary unit tomeet such requirement.
 35. A detection method as claimed in claim 1,further comprising, following detection of one or more secondary unitsnot requiring power in the absence of one or more secondary unitsrequiring power, restricting or stopping inductive power supply from theprimary unit.
 36. A primary unit for use in an inductive power transfersystem, the primary unit being operable to transmit power wirelessly byelectromagnetic induction to at least one secondary unit of the systemlocated in proximity to the primary unit and/or to a foreign objectlocated in said proximity, said at least one secondary unit including aregulator with an operating threshold, the primary unit comprising:driving means operable to transmit power sufficient to operate theregulator at or above the operating threshold, the driving meansoperable to drive the primary unit so that in a driven state themagnitude of an electrical drive signal supplied to one or more primarycoils of the primary unit changes from a first value to a second value,wherein power transmitted at the first value is sufficient to operatethe regulator at or above the operating threshold and power transmittedat the second value is sufficient to operate the regulator at or abovethe operating threshold; means for assessing the effect of such drivingon an electrical characteristic of the primary unit based on adifference between a first measurement of the electrical characteristicat the first value and a second measurement of the electricalcharacteristic at the second value; and means for detecting independence upon the assessed effect the presence of a said secondaryunit including said regulator and/or a foreign object located inproximity to said primary unit.
 37. A primary unit as claimed in claim36, wherein the electrical characteristic of the primary unit is a levelof power being drawn from the primary unit.
 38. An inductive powertransfer system, comprising a primary unit and at least one secondaryunit, said at least one secondary unit including a secondary coil, theprimary unit being operable to transmit power wirelessly byelectromagnetic induction to at least one said secondary unit located inproximity to the primary unit and/or to a foreign object located in saidproximity, the system comprising: means for regulating such that a powerdrawn from said secondary coil at or above an operating threshold is aknown function of an input level; driving means operable to transmitpower sufficient to operate the means for regulating at or above theoperating threshold, said driving means operable to drive the primaryunit so that in a driven state a magnitude of an electrical drive signalsupplied to one or more primary coils of the primary unit changes from afirst value to a second value, wherein power transmitted at the firstvalue is sufficient to operate the regulator at or above the operatingthreshold and power transmitted at the second value is sufficient tooperate the regulator at or above the operating threshold; means forassessing the effect of such driving on an electrical characteristic ofthe primary unit based on a difference between a first measurement ofthe electrical characteristic at the first value and a secondmeasurement of the electrical characteristic at the second value; meansfor detecting in dependence upon the assessed effect the presence of asaid secondary unit and/or a foreign object located in proximity to saidprimary unit.
 39. An inductive power transfer system as claimed in claim38, wherein the electrical characteristic of the primary unit is a levelof power being drawn from the primary unit.
 40. An inductive powertransfer system as claimed in claim 39, wherein the or each saidsecondary unit of the system is configured such that, when in proximityto the primary unit and receiving power inductively therefrom, anelectrical characteristic of that secondary unit responds to suchdriving in an expected manner, the detecting means being operable todetermine whether a said secondary unit and/or a foreign object ispresent in proximity to said primary unit in dependence upon results ofsuch assessment and the or each such expected response.
 41. An inductivepower transfer system as claimed in claim 40, wherein for the or eachsaid secondary unit the electrical characteristic of that secondary unitis its power drawn from the primary unit.
 42. An inductive powertransfer system as claimed in claim 41, wherein, for the or at least onesaid secondary unit, the expected response is that its electricalcharacteristic does not substantially change in response to the drivingthe primary unit so that in a driven state the magnitude of anelectrical drive signal supplied to one or more primary coils of theprimary unit changes from a first value to a second value.